1. Field of the Invention
The invention relates to the field of audio equalizers and tone control.
2. Prior Art
Audio equalizers are well-known in the art for boosting and cutting predetermined frequency ranges or bands. Ideally, an equalizer should have reciprocal, mirror image characteristics for boosting when compared to cutting. Moreover, an ideal equalizer should provide an infinite-depth notch.
Ideal equalizers, if connected in cascade, should be capable of providing a flat, a overall complex gain response since the boosting of one equalizer should exactly cancel the cutting of the other. Looked at in another way, the first equalizer's frequency response precisely complements the second's, and cancels out the effect of the first equalizer. To have this reciprocity, the zeros of one equalizer must equal the poles of the other equalizer and vice-versa. (The complex gain is generally described as a function of the Laplace transform complex variable "s", as is well-known in the art. In the description which follows, the "poles" and "zeros" refer to the poles and zeros of transmission, respectively.)
Many equalizers well-known in the art achieve equalization by combining the output of a filter with the filter's input to amplify or attenuate signals in a frequency-selective way. If the filter is a first-order low-pass filter [having a complex gain function of the form p/(s+p)], it produces a so-called "low-pass shelving" characteristic. If the filter is a first-order high pass filter [having a complex gain function of the form s/(s+p)], it produces a so-called "high-pass shelving" characteristic. If the filter is a second-order band-pass filter [having a complex gain function of the form Qs/(s.sup.2 .degree.Qs+.omega..sub.o)], it produces a so-called "peaking" or "dipping" characteristic, with associated bell-shaped curves of gain versus frequency.
There are two widely used circuits for combining the output of the filter with its input to achieve the equalization. These circuits are shown in FIGS. 1 and 2 and are subsequently discussed in conjunction with these figures.
As will be seen, the present invention provides improved characteristics in that the boosting and cutting are reciprocal and since an infinite-depth cut is obtained.